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The present invention relates to a modulator assembly for modulation of light, in particular for use as part of an opto-electronic communication network.
In an opto-electronic communication network, it is desirable to produce high frequency modulated light. One way of achieving this is to pass light of initially constant intensity through a modulator. Normally the modulator is formed using a modulator medium whose optical properties depend on an electric field applied across it, so that modulating the electric field across the modulator medium results in a modulation in the intensity of light passing through it. Examples of modulators used to modulate light from a laser include electro-absorption modulators, Mach-Zender interferometer modulators, and Fabry-Perot modulators.
The modulator is normally biased with a quiescent voltage in order to bring it into an operating mode where an absorption edge of the modulator medium is close to the wavelength of the light being modulated. A modulation voltage is superimposed onto the quiescent voltage in order to modulate the intensity of light passing through the modulator medium. The quiescent voltage normally being constant in time or slowly varying compared to the modulation voltage since it need only change with changing material properties of the modulator medium e.g. with temperature.
One of the disadvantages of such a system is that the absorption/transmission spectra of materials usually suitable for modulator media change with temperature, owing to changes in the semiconductor bandgap of the modulator material, changes in the cavity refractive index or changes in the geometrical thickness of the device. For example, the absorption edge of an electro-absorption modulator is known to change by around 0.4 nm/K whereas the operating wavelength of a typical single mode distributed feedback lasers changes only at around 0.07 nm/K.
Conventionally, for successful operation such devices are cooled to maintain alignment of the modulator absorption edge to that of the lasing wavelength. This is often accomplished using a thermoelectric cooler (TEC) to dissipate the heat generated by the laser diode. Such heat dissipation consumes large amounts of power and requires a large heatsinking capability.
Recently laser diodes have been fabricated which do not themselves require cooling for successful operation, so it is highly desirable to develop a modulator assembly which allows a laser and modulator combination to operate without the need for a TEC.
According to one aspect of the present invention there is provided a light modulator assembly comprising a modulator element comprising a modulating medium, for modulating the intensity of light passing therethrough; and means for applying an electric field across the modulating medium, the modulating medium having an absorption edge at a wavelength dependent upon the temperature of the medium and upon the applied electric field; and means for heating the modulating medium such that the wavelength of the absorption edge aligns with the wavelength of the light to be modulated. The means for applying an electric field comprises an electrode, which also constitutes the means for heating the modulating medium.
Preferably the means for heating the modulating medium comprises a metallic element in close proximity to the modulating element.
The device may also comprise a thermally insulating layer of low thermal conductivity and my also have slots each side of the modulating medium.
Another aspect of the invention relates to a light assembly comprising a modulator element comprising a modulating medium, for modulating the intensity of light passing therethrough; and means for applying an electric filed across the modulating medium, the modulating medium having an absorption range at a wavelength dependent on the temperature of the medium and upon the applied electric field; and means for heating the modulating medium such that the wavelength of the absorption edge aligns with the wavelength of the light to be modulated, and wherein the extinction ratio of an induced photocurrent in the modulating element is used in a feedback loop to set the drive current of the heating element.
Another aspect of the invention relates to a light modulator assembly comprising a modulator element comprising a modulating medium for modulating the intensity of a light passing therethrough the modulating medium having an absorption edge at a wavelength dependent upon temperature of the medium and upon electric field applied to the medium, and an electrode positioned applied arranged for applying an electric field to the medium and for heating the medium such that the wavelength of the absorption edge aligns with the wavelength of the light to be modulated.
An added aspect of the invention relates to a light modulator assembly comprising a modulator element comprising a modulating medium, for modulating the intensity of light passing therethrough; and an electric field applicator for applying an electric field across the modulating medium, the modulating medium having an absorption edge at a wavelength dependent upon the temperature of the medium and upon the applied electric field; and a heater for heating the modulating a feedback loop coupled to be responsive to the extinction ratio of an induced photocurrent in the modulating element for controlling drive current of the heating element.